Message-ID: <CAGsJ_4x3Aj7wieK1FQKQC4Vbz5N+1dExs=Q70KQt-whS1dMxpw@mail.gmail.com>
Date: Fri, 20 Dec 2024 16:09:41 +1300
From: Barry Song <21cnbao@...il.com>
To: Chen Ridong <chenridong@...weicloud.com>, cuibixuan@...o.com
Cc: akpm@...ux-foundation.org, mhocko@...e.com, hannes@...xchg.org, 
	yosryahmed@...gle.com, yuzhao@...gle.com, david@...hat.com, 
	willy@...radead.org, ryan.roberts@....com, wangkefeng.wang@...wei.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, chenridong@...wei.com, 
	wangweiyang2@...wei.com, xieym_ict@...mail.com
Subject: Re: [PATCH -next v5] mm: vmscan: retry folios written back while
 isolated for traditional LRU

On Fri, Dec 20, 2024 at 3:30 PM Barry Song <21cnbao@...il.com> wrote:
>
> On Fri, Dec 20, 2024 at 2:19 PM Chen Ridong <chenridong@...weicloud.com> wrote:
> >
> > From: Chen Ridong <chenridong@...wei.com>
> >
> > The page reclaim isolates a batch of folios from the tail of one of the
> > LRU lists and works on those folios one by one.  For a suitable
> > swap-backed folio, if the swap device is async, it queues that folio for
> > writeback.  After the page reclaim finishes an entire batch, it puts back
> > the folios it queued for writeback to the head of the original LRU list.
> >
> > In the meantime, the page writeback flushes the queued folios also by
> > batches.  Its batching logic is independent from that of the page reclaim.
> > For each of the folios it writes back, the page writeback calls
> > folio_rotate_reclaimable() which tries to rotate a folio to the tail.
> >
> > folio_rotate_reclaimable() only works for a folio after the page reclaim
> > has put it back.  If an async swap device is fast enough, the page
> > writeback can finish with that folio while the page reclaim is still
> > working on the rest of the batch containing it.  In this case, that folio
> > will remain at the head and the page reclaim will not retry it before
> > reaching there.
> >
> > The commit 359a5e1416ca ("mm: multi-gen LRU: retry folios written back
> > while isolated") only fixed the issue for mglru. However, this issue
> > also exists in the traditional active/inactive LRU. This issue will be
> > worse if THP is split, which makes the list longer and needs longer time
> > to finish a batch of folios reclaim.
> >
> > This issue should be fixed in the same way for the traditional LRU.
> > Therefore, the common logic was extracted to the 'find_folios_written_back'
> > function firstly, which is then reused in the 'shrink_inactive_list'
> > function. Finally, retry reclaiming those folios that may have missed the
> > rotation for traditional LRU.
> >
> > Link: https://lore.kernel.org/linux-kernel/20241010081802.290893-1-chenridong@huaweicloud.com/
> > Link: https://lore.kernel.org/linux-kernel/CAGsJ_4zqL8ZHNRZ44o_CC69kE7DBVXvbZfvmQxMGiFqRxqHQdA@mail.gmail.com/
> > Signed-off-by: Chen Ridong <chenridong@...wei.com>
> > ---
> >  mm/vmscan.c | 108 ++++++++++++++++++++++++++++++++++------------------
> >  1 file changed, 70 insertions(+), 38 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 39886f435ec5..e67e446540ba 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -283,6 +283,39 @@ static void set_task_reclaim_state(struct task_struct *task,
> >         task->reclaim_state = rs;
> >  }
> >
> > +/**
> > + * find_folios_written_back - Find and move the written back folios to a new list.
> > + * @list: filios list
> > + * @clean: the written back folios list
> > + * @is_retried: whether the list has already been retried.
> > + */
> > +static inline void find_folios_written_back(struct list_head *list,
> > +               struct list_head *clean, bool is_retried)
> > +{
> > +       struct folio *folio;
> > +       struct folio *next;
> > +
> > +       list_for_each_entry_safe_reverse(folio, next, list, lru) {
> > +               if (!folio_evictable(folio)) {
> > +                       list_del(&folio->lru);
> > +                       folio_putback_lru(folio);
> > +                       continue;
> > +               }
> > +
> > +               /* retry folios that may have missed folio_rotate_reclaimable() */
> > +               if (!is_retried && !folio_test_active(folio) && !folio_mapped(folio) &&
> > +                   !folio_test_dirty(folio) && !folio_test_writeback(folio)) {
> > +                       list_move(&folio->lru, clean);
> > +                       continue;
> > +               }
> > +
> > +               /* don't add rejected folios to the oldest generation */
> > +               if (lru_gen_enabled() && !lru_gen_distance(folio, false))
> > +                       set_mask_bits(&folio->flags, LRU_REFS_FLAGS, BIT(PG_active));
> > +       }
> > +
> > +}
> > +
> >  /*
> >   * flush_reclaim_state(): add pages reclaimed outside of LRU-based reclaim to
> >   * scan_control->nr_reclaimed.
> > @@ -1959,14 +1992,18 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
> >                 enum lru_list lru)
> >  {
> >         LIST_HEAD(folio_list);
> > +       LIST_HEAD(clean_list);
> >         unsigned long nr_scanned;
> > -       unsigned int nr_reclaimed = 0;
> > +       unsigned int nr_reclaimed, total_reclaimed = 0;
> > +       unsigned int nr_pageout = 0;
> > +       unsigned int nr_unqueued_dirty = 0;
> >         unsigned long nr_taken;
> >         struct reclaim_stat stat;
> >         bool file = is_file_lru(lru);
> >         enum vm_event_item item;
> >         struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> >         bool stalled = false;
> > +       bool is_retried = false;

The name is_retried is a bit confusing. It should be is_retry or
is_retrying since
we are currently retrying, not that we have already retried.

> >
> >         while (unlikely(too_many_isolated(pgdat, file, sc))) {
> >                 if (stalled)
> > @@ -2000,22 +2037,47 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
> >         if (nr_taken == 0)
> >                 return 0;
> >
> > +retry:
> >         nr_reclaimed = shrink_folio_list(&folio_list, pgdat, sc, &stat, false);
> >
> > +       sc->nr.dirty += stat.nr_dirty;
> > +       sc->nr.congested += stat.nr_congested;
> > +       sc->nr.unqueued_dirty += stat.nr_unqueued_dirty;
> > +       sc->nr.writeback += stat.nr_writeback;
> > +       sc->nr.immediate += stat.nr_immediate;
> > +       total_reclaimed += nr_reclaimed;
> > +       nr_pageout += stat.nr_pageout;
> > +       nr_unqueued_dirty += stat.nr_unqueued_dirty;
> > +
> > +       trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
> > +                       nr_scanned, nr_reclaimed, &stat, sc->priority, file);
>
> This is a bit odd, as nr_scanned during a retry still uses the
> previous nr_scanned
> value. However, I find that mglru shows no difference.
>
> retry:
>         reclaimed = shrink_folio_list(&list, pgdat, sc, &stat, false);
>         sc->nr.unqueued_dirty += stat.nr_unqueued_dirty;
>         sc->nr_reclaimed += reclaimed;
>         trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
>                         scanned, reclaimed, &stat, sc->priority,
>                         type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
>
> Currently, the active/inactive state aligns with mglru in this trace.
> It seems that
> the userspace BPF should recognize that the nr_scanned during a retry doesn't
> indicate we are isolating new nr_scanned folios. Ideally, the is_retry
> flag should
> be passed to the trace, allowing userspace to identify that it's a retry and
> disregard the nr_scanned value.
>
> It might be worth addressing this in a separate patch. Add Bixuan to clarify
> how userspace depends on this trace and if "retry" will break his userspace
> BPF for both MGLRU and active/inactive cases.
>
> Otherwise, the patch looks good to me.
>

By the way, it's completely clear that the trace was added after mglru's retry:
https://lore.kernel.org/linux-mm/20240105013607.2868-3-cuibixuan@vivo.com/

Therefore, I don't believe the potential confusion about nr_scanned in the trace
should prevent Ridong's fix for the missed rotation of written-back folios from
proceeding.

If there is an issue with that, we should open a separate thread to address the
trace.

Please feel free to add the below in the future version after you fix
"is_retried".

Reviewed-by: Barry Song <baohua@...nel.org>

> > +
> > +       find_folios_written_back(&folio_list, &clean_list, is_retried);
> > +
> >         spin_lock_irq(&lruvec->lru_lock);
> >         move_folios_to_lru(lruvec, &folio_list);
> >
> >         __mod_lruvec_state(lruvec, PGDEMOTE_KSWAPD + reclaimer_offset(),
> >                                         stat.nr_demoted);
> > -       __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
> >         item = PGSTEAL_KSWAPD + reclaimer_offset();
> >         if (!cgroup_reclaim(sc))
> >                 __count_vm_events(item, nr_reclaimed);
> >         __count_memcg_events(lruvec_memcg(lruvec), item, nr_reclaimed);
> >         __count_vm_events(PGSTEAL_ANON + file, nr_reclaimed);
> > +
> > +       if (!list_empty(&clean_list)) {
> > +               list_splice_init(&clean_list, &folio_list);
> > +               is_retried = true;
> > +               spin_unlock_irq(&lruvec->lru_lock);
> > +               goto retry;
> > +       }
> > +       __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
> >         spin_unlock_irq(&lruvec->lru_lock);
> > +       sc->nr.taken += nr_taken;
> > +       if (file)
> > +               sc->nr.file_taken += nr_taken;
> >
> > -       lru_note_cost(lruvec, file, stat.nr_pageout, nr_scanned - nr_reclaimed);
> > +       lru_note_cost(lruvec, file, nr_pageout, nr_scanned - total_reclaimed);
> >
> >         /*
> >          * If dirty folios are scanned that are not queued for IO, it
> > @@ -2028,7 +2090,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
> >          * the flushers simply cannot keep up with the allocation
> >          * rate. Nudge the flusher threads in case they are asleep.
> >          */
> > -       if (stat.nr_unqueued_dirty == nr_taken) {
> > +       if (nr_unqueued_dirty == nr_taken) {
> >                 wakeup_flusher_threads(WB_REASON_VMSCAN);
> >                 /*
> >                  * For cgroupv1 dirty throttling is achieved by waking up
> > @@ -2043,18 +2105,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
> >                         reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
> >         }
> >
> > -       sc->nr.dirty += stat.nr_dirty;
> > -       sc->nr.congested += stat.nr_congested;
> > -       sc->nr.unqueued_dirty += stat.nr_unqueued_dirty;
> > -       sc->nr.writeback += stat.nr_writeback;
> > -       sc->nr.immediate += stat.nr_immediate;
> > -       sc->nr.taken += nr_taken;
> > -       if (file)
> > -               sc->nr.file_taken += nr_taken;
> > -
> > -       trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
> > -                       nr_scanned, nr_reclaimed, &stat, sc->priority, file);
> > -       return nr_reclaimed;
> > +       return total_reclaimed;
> >  }
> >
> >  /*
> > @@ -4585,12 +4636,10 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
> >         int reclaimed;
> >         LIST_HEAD(list);
> >         LIST_HEAD(clean);
> > -       struct folio *folio;
> > -       struct folio *next;
> >         enum vm_event_item item;
> >         struct reclaim_stat stat;
> >         struct lru_gen_mm_walk *walk;
> > -       bool skip_retry = false;
> > +       bool is_retried = false;
> >         struct lru_gen_folio *lrugen = &lruvec->lrugen;
> >         struct mem_cgroup *memcg = lruvec_memcg(lruvec);
> >         struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> > @@ -4616,24 +4665,7 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
> >                         scanned, reclaimed, &stat, sc->priority,
> >                         type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
> >
> > -       list_for_each_entry_safe_reverse(folio, next, &list, lru) {
> > -               if (!folio_evictable(folio)) {
> > -                       list_del(&folio->lru);
> > -                       folio_putback_lru(folio);
> > -                       continue;
> > -               }
> > -
> > -               /* retry folios that may have missed folio_rotate_reclaimable() */
> > -               if (!skip_retry && !folio_test_active(folio) && !folio_mapped(folio) &&
> > -                   !folio_test_dirty(folio) && !folio_test_writeback(folio)) {
> > -                       list_move(&folio->lru, &clean);
> > -                       continue;
> > -               }
> > -
> > -               /* don't add rejected folios to the oldest generation */
> > -               if (!lru_gen_distance(folio, false))
> > -                       set_mask_bits(&folio->flags, LRU_REFS_FLAGS, BIT(PG_active));
> > -       }
> > +       find_folios_written_back(&list, &clean, is_retried);
> >
> >         spin_lock_irq(&lruvec->lru_lock);
> >
> > @@ -4656,7 +4688,7 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
> >         list_splice_init(&clean, &list);
> >
> >         if (!list_empty(&list)) {
> > -               skip_retry = true;
> > +               is_retried = true;
> >                 goto retry;
> >         }
> >
> > --
> > 2.34.1
> >
>

Thanks
barry

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